In screw extruders for processing rubber mixtures, feed rollers have been used to facilitate the feeding of the rubber mixture to the extruder screw. However, their use has always been problematic because of the danger of the mixture being forced into the sealing spaces, bearings and drive of the feed roller. If the feed roller is not thoroughly and regularly cleaned, in particular during interruptions in operation of the extruder, the pressed-in parts of the mixture become hardened or vulcanized and block the feed roller when operation of the machine is resumed and this frequently leads to serious damage.
Despite these problems, feed rollers have been adopted as standard, in particular for recently developed screw extruders for cold feeding of rubber mixtures, because they effect a partial plasticizing of hard viscous rubber mixtures, which is considered very important. Extruders for processing silicone without feed rollers are unthinkable.
For the above mentioned reasons, the sealing of the feed roller in its mounting in the extruder is highly important, in particular on account of the unavoidable phenomenon of wear during operation. An essential point in this connection is the continual cleaning of the feed roller with the help of a scraping knife which is usually adjustable. In particular with cold working screw extruders, there are large forces created by the plasticizing work between the screw and the feed roller which are sufficient elastically to bend the roller cyclicly during operation. This elastic deformation produces cyclicly an insufficient pressing force of the scraping knife or a minute gap between the surface of the feed roller and the stationary scraping knife through which there is formed on the roller a deposit which, because of the high temperature of the roller, is quickly vulcanized. This can be a very serious disadvantage and must be avoided.
A frequently used cooling of the feed roller, which is necessary for thermic reasons for particular rubber mixtures, does not solve the problem of the danger of vulcanization of material adhering to the feed roller.
As U.S. Pat. No. 1,422,561 shows, it has already been attempted in 1920 to attain continual cleaning of the feed roller surface through spring pressing of the scraping knife with a constant pressing force. For this it is assumed that the scraping knife remains easily movable during the operation which, because of the rubber mixture being pressed into the smallest space, cannot be assured. Also essentially stronger springs are not sufficient to guarantee movement and have also the disadvantage that the strong pressing of the scraping knife against the feed roller results in friction and wear of the knife and/or roller. The same is true for strong pressing of the scraping knife through pneumatic, hydraulic or electrically generated force.
As DE-PS 39 24 734 shows, it was attempted to solve the scraping knife sealing problem by applying a constant pressing force to the scraping knife. There was provided a very expensive force measuring system on the feed roller bearings with which the constantly changing loading of the feed roller by the strip or granular infeed between the screw and the feed roller mantle in the feeding region is measured. According to this variable loading, the pressing force is held constant. Also here it is assumed that the scraping knife is easily movable i.e. is not blocked by fouling or through vulcanization of the rubber mixture. This is achieved in short laboratory test but not in actual practice. Thus the actual problem of cleaning the feed roller is not solved by applying a constant pressing force to the scraping knife. When the feed roller is elastically deflected by reason of high mass pressure, the pressing force exerted on the scraping knife must be greater than when no, or only limited, pressure is exerted on the feed roller. Hence a constant pressing force which is oriented to the highest sealing load of the knife is very objectionable in the sense of avoiding abrasion. This construction is hence not only very expensive but also not practically meaningful.
A scraping knife construction which is not based on the premise of easy movability for a constant pressing of the scraping knife on the roller is disclosed in U.S. Pat. No. 2,537,395. Here the knife is not arranged at an angle but is substantially tangential to the feed roller. This construction stems from a time when cold feeding with the resultant high forces on the feed roller was not yet known. With warm feeding of the extruder, which was usual at that time, the extruder was exclusively fed with prewarmed rubber mixture which exerted only limited forces between the screw and the feed roller. The roller served only to assist in feeding and not, on the contrary, for preplasticizing the mixture. The knife construction and arrangement was hence fully rigid as an elastic deformation of the roller was practically not existent and hence a scraping knife set in optimal position prior to putting the extruder into operation satisfied the requirements of that time.